In the bias-ply automotive pneumatic tire, carcass-protective reinforcing layers, known as breakers, are interposed between the carcass and the tread. In the radial-ply tire, reinforcing layers called "belts" are interposed between the carcass and the tread and serve to brace up the carcass in the radial direction. In such a pneumatic tire, it is common practice to increase the service life of the tire by embedding steel cords in the breakers or belts.
This invention relates to a pneumatic tire in which the rubber layer, i.e. said breaker or belt, between the carcass and tread has been reinforced with steel cords.
The breaker or belt is often constructed in a plurality of layers. Particularly when the anti-cut property of a pneumatic tire must be increased, a plurality of steel cords capable of sufficient elongation are embedded at appropriate spacings in the outermost, i.e. adjacent to the tread, layer of the aforementioned breaker or belt.
In the conventional steel cord, it is common practice, as explained below, to implement a large elongation value by adopting a multi-strand structure.
FIGS. 9 and 10 are cross-section views showing the multi-strand steel cords heretofore used in the conventional pneumatic tire; thus, FIG. 9 represents the 4.times.4.times.0.23 construction, while FIG. 10 represents the 3.times.7.times.0.22 construction.
Referring to FIG. 9, a steel cord 10 is an assembly of four strands 16 twisted together and each strand 16, in turn, is an assembly of four constituent wires 12, each constituent wire 12 being a steel wire with a diameter of 0.23 mm. The twisting pitch of these constituent wires 12 is 3.5 mm and that of the strands 16 is 5.5 mm.
The steel cord 10 illustrated in FIG. 10 is an assembly of three strands 16 twisted together. Each strand 16 is an assembly of seven constituent wires 12 and each of the wires 12 is a steel wire with a diameter of 0.22 mm. The twisting pitch of the wires 12 is 4.0 mm and that of the strands 16 is 7.5 mm.
Since each of these steel cords 10 is a low-pitch multi-ply structure, it has a large elongation and is flexible, insuring a large impact-absorbing or damping capacity. Therefore, the conventional pneumatic tires utilizing these steel cords 10 had high anti-cut properties.
The conventional tires employing steel cords of the above-described constructions present the following problems, however.
Thus, in the conventional multi-strand steel cords 10, the degree of so-called "twisting loss" of tenacity or strength is high and the strength of the respective wires 12 cannot be effectively utilized. In order to obtain a desired strength for steel cord 10, an increased number of constituent wires 12 is required but the use of so many wires 12 adds to the overall weight of the steel cord 10. Moreover, since the steel cord 10 used heretofore is a flexible steel cord, the pneumatic tire is markedly deformed on the tread to create a large rolling resistance, thus detracting from milage.
Furthermore, as will be apparent from FIGS. 9 and 10, the wires 12 in the conventional construction are round in section and disposed in close contact and, for this reason, a closed air space 18 is created in the approximate center of the constituent wires. Therefore, in this steel cord 10, rubber does not easily enter and fill the space 18. In other words, the resulting breaker or belt will have the space 18 unfilled with rubber. If the tread of the tire is damaged and consequently water finds its way from the site of injury into the space 18, the invading water migrates within the space 18 along the length of steel cord 10 and, also, remains entrapped therein. Consequently the steel cord 10 becomes rusted with time, detracting from the strength of bond to the rubber. If this decrease in bond strength progresses, there occurs the trouble known as "separation".
Therefore, the present applicant previously proposed, in the copending Japanese Patent Application No. 79399/1988, "a pneumatic tire in which the rubber layer between the carcass and tread has been reinforced with single-strand steel cords with an elongation at break of not less than 4%".
FIG. 11 is a cross-section view showing the steel cord used in the pneumatic tire according to the above patent application which remains yet to be laid open.
This steel cord 10 is a single-strand cord of 1.times.5.times.0.38 construction. Thus, it is an assembly of five wires 12 each having a diameter of 0.38 mm. The twisting pitch is 6.5 mm and the elongation at break of the cord is 6.5%.
Despite being a single-strand cord, this steel cord 10 has an elongation at break of not less than 4% and, therefore, offers an anti-cut performance comparable to that of a multi-strand steel cord. Furthermore, because of the very single-strand construction, the rate of wire tenacity utilization is high. This means that the desired tire strength is attainable even if the overall weight of the tire cord is decreased, and that accordingly a lightweight pneumatic tire can be implemented. Moreover, because of the adequate flexural hardness of the steel cord 10, the rigidity of the pneumatic tire is enhanced. Therefore, the deformation and, hence, rolling resistance of the tire are reduced, thus contributing to milage. Furthermore, since clearances are created between constituent wires 12 at intervals along the length of the steel cord, the space 14 enclosed by the wires 12 is exposed to the atmosphere. Therefore, in the vulcanization of the pneumatic tire having a breaker or belt in which this steel cord 10 has been embedded, the rubber finds its way readily into the space 14 and as the space is thus filled with the rubber, the incidence of "separation" is virtually inhibited.
However, even in the case of a steel cord having the same 1.times.5.times.0.38 construction, its anti-cut property is poor when its twisting pitch is as large as 18.0 mm and its elongation is as small as, for example, 2.5%. In addition, as illustrated in FIG. 12, a closed space 18 is formed by and among the wires 12. Therefore, in the result of arresting the entry of rubber into the space 18, this cord is equivalent to the aforementioned multi-strand cord.
In the pneumatic tire disclosed in the copending patent application mentioned above, the wires 12 are identical in diameter and shaping rate so that all the wires 12 form one common circumcircle 22. Therefore, though the incidence of separation is inhibited, the effect of anchoring the steel cord in the rubber constituting the breaker or belt is so small that the separation once started tends to grow easily.
Having been accomplished in view of the above situation, this invention has as its object to provide a pneumatic tire in which the strength or tenacity of material wires is effectively exploited, while maintaining the anti-cut property of the tire at a level comparable to that of the conventional tire, to thereby improve the rigidity of the tire while the incidence and growth of separation having been successfully inhibited.